This invention relates to amplifiers, and more specifically, to a high fidelity audio amplifier or preamplifier system incorporating a floating bridge circuit topology.
A floating bridge or “circlotron” amplifier includes a floating bridge output section in which a pair of voltage follower devices or circuits and a pair of floating DC power supplies are arranged as a balanced bridge. Practical amplifier systems of this type also generally include a preceding voltage gain section to raise an applied input signal voltage to the required output level and to supply a differential drive signal to the output section. Ideally, the included voltage gain section provides linear gain over a wide range, has ground-referenced input terminals to allow for direct coupling of a signal source, and is simple and inexpensive to implement. A long-tail pair differential amplifier meets these criteria, and can be combined with a floating bridge output section to form an inherently linear amplifier system. Such an amplifier system is highly desirable, as it obviates the need for large amounts of global negative feedback to reduce amplifier distortion. It is known in the art that heavy use of global negative feedback can lead to poor transient response, marginal stability, and increased high-order residual distortion products, all of which can degrade perceived amplifier sound quality.
Although floating bridge amplifier systems have been used successfully in audio applications in the past, much room remains for improvement of this topology. Significant drawbacks and limitations of previous embodiments include:                a) The need for both floating and fixed polarity DC power supplies. In addition to the two floating DC power sources required by the output section, positive and negative fixed polarity power supplies are needed to operate a differential voltage gain section with direct input coupling. Because standard transformers for analog power supplies typically have a single pair of secondary windings, providing two floating and two fixed polarity DC power supplies generally requires the use of either two standard transformers or a single unit of custom design. Power transformers typically are among the most costly components in an amplifier, so either of these solutions adds significant cost, and may also complicate power supply wiring and fusing.        b) The need for a Class A output stage. Some previous floating bridge amplifier designs using little or no global negative feedback rely on the continuous conduction of both output voltage followers to ensure low overall distortion and consistent control of the voltage across the load. Such continuous conduction, known as class A operation, draws significant current from the power supplies even at idle. This mandates more rigorous and expensive thermal management for the output section, and necessitates the use of heavy-duty power supply components to avoid excessive voltage sag, heating, and noise. These requirements contribute to a design that is more complex and costly to build, and more expensive and inconvenient to operate due to its increased bulk, power consumption, and waste heat.        c) The requirement to isolate the voltage gain section from power supply noise and modulation in order to maintain high linearity and low noise. Some previous floating bridge designs simplify power supply requirements by operating one or more amplifier voltage gain stages from the floating output stage power supplies through passive decoupling networks and bootstrap circuits. The decoupling networks offer effective noise isolation at the cost of reduced operating voltage, potentially limiting the maximum output signal swing. The problem of limited signal swing is overcome by adding resistive bootstrap circuits, but these circuits couple power supply noise and modulation into the signal path, and thus work against the goal of high open-loop performance.        
Thus, it is apparent that the use of floating bridge amplifier systems in audio, while highly desirable, still entails significant limitations and drawbacks. The present invention provides a floating bridge amplifier system that largely overcomes these impediments and provides additional benefits, as further described below.
Several objects of the present invention are:                To provide a floating bridge amplifier system whose power requirements can be met by a power transformer of standard design, i.e., one having a single pair of independent secondary windings.        To provide a floating bridge amplifier system in which the signal path is kept linear and the output substantially free of power supply noise and modulation effects.        To provide a floating bridge amplifier system whose output section exhibits distortion and load control comparable to or better than a class A output section, but with lower idle current, heat, bulk, and cost than a class A amplifier.        
Further objects and advantages of this invention will become apparent from the following descriptions of the preferred embodiments.